Liquid crystal display devices have been used for, for example, clocks and watches, electronic calculators, various home electric appliances, measurement apparatuses, automotive panels, word processors, electronic notebooks, printers, computers, and television sets. Typical examples of a liquid crystal display mode include a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a dynamic light scattering (DS) mode, a guest-host (GH) mode, an in-plane switching (IPS) mode, an optical compensated birefringence (OCB) mode, an electrically controlled birefringence (ECB) mode, a vertical alignment (VA) mode, a color super homeotropic (CSH) mode, and a ferroelectric liquid crystal (FLC). Examples of a driving method include static driving, multiplex driving, a passive matrix method, and an active matrix (AM) method in which driving is performed with a thin-film transistor (TFT) or a thin-film diode (TFD).
Among these display modes, for example, the IPS mode, the ECB mode, the VA mode, or the CSH mode is characterized by using a liquid crystal material that has negative Δε. Of these, in particular, the VA display mode driven by AM driving is used for applications, such as television sets, including display elements required to have high speeds and wide viewing angles.
Nematic liquid crystal compositions used for, for example, the VA display mode, are required to have low-voltage driving, a fast response, and a wide operating temperature range. That is, such liquid crystal compositions are each required to have negative Δε whose absolute value is large, low viscosity, and a high nematic-isotropic liquid phase transition temperature (Tni). In view of the setting of Δn×d, which is the product of refractive index anisotropy (Δn) and a cell gap (d), Δn of a liquid crystal material needs to be adjusted within an appropriate range, depending on the cell gap. In addition, when a liquid crystal display element is used for, for example, a television set, emphasis is placed on a fast response. Thus, a liquid crystal material having low viscosity (η) is required.
Hitherto, various compounds each having negative Δε whose absolute value is large have been studied to improve the characteristics of liquid crystal compositions.
A liquid crystal composition containing compounds (A) and (B) with a 2,3-difluorophenylene skeleton described below (see Patent Literature 1) is disclosed as a liquid crystal material having negative Δε.

The liquid crystal composition contains compounds (C) and (D) serving as compounds having Δε of substantially zero. In the case of the liquid crystal composition, however, sufficiently low viscosity is not achieved for a liquid crystal composition for use in, for example, a liquid crystal television set required to have a fast response.

Liquid crystal compositions each containing a compound represented by formula (E) have already been disclosed and include a liquid crystal composition which contains compound (D) in combination with it and which has low Δn (see Patent Literature 2); and a liquid crystal composition to which a compound (alkenyl compound), such as compound (F), containing an alkenyl group in its molecule is added to improve its response speed (see Patent Literature 3). To achieve both high Δn and high reliability, further studies have been required.

A liquid crystal composition containing a compound represented by formula (G) has already been disclosed (see Patent Document 4). This liquid crystal composition is also a liquid crystal composition that contains a compound containing an alkenyl compound, such as compound (F) described above. Thus, display defects, such as image persistence and display unevenness, are disadvantageously liable to occur.

The influence of a liquid crystal composition containing an alkenyl compound on display defects has been already disclosed (see Patent Literature 5). However, in general, a reduction in alkenyl compound content increases η of the liquid crystal composition to cause difficulty in achieving a fast response. It is thus difficult to achieve both of the suppression of the display defects and the fast response.
As described above, it has been difficult to develop a liquid crystal composition which has negative Δε, which achieves both high Δn and low η, and which has no or minimal display defects only by the combinations of the compound having negative Δε with compounds (C), (D), and (F).
A liquid crystal composition in which formulae (A) and (G) are combined with formula (III-F31) having Δε of substantially zero is disclosed (see Patent Literature 6). However, in a production process of a liquid crystal display element, a compound with a low vapor pressure is vaporized at an extremely low pressure during the injection of a liquid crystal composition into a liquid crystal cell, so it was seemingly impossible to increase the content of the compound. Thus, in the liquid crystal composition, the content of formula (III-F31) is limited. Although the liquid crystal composition has Δn, the liquid crystal composition disadvantageously has significantly high viscosity.

In Patent Literatures 6 and 7, liquid crystal compositions containing compounds having fluorine-substituted terphenyl structures have already been disclosed.
Patent Literature 8 discloses that the use of a liquid crystal material having a large index (FoM) represented by (expression 1) improves the response speed of a homeotropic liquid crystal cell. However, the improvement in the response speed of the liquid crystal composition described in the specification is not sufficient.[Math. 1]FoM=K33·Δn2/γ1  (expression 1)                K33: elastic constant        Δn: refractive index anisotropy        γ1: rotational viscosity        
Thus, a liquid crystal composition used for, for example, liquid crystal television sets required to have a fast response has been required to have a sufficiently low solid-nematic phase transition temperature (Tcn), sufficiently low viscosity (η), sufficiently low rotational viscosity (γ1), and a large elastic constant (K33) without reducing the refractive index anisotropy (Δn) or the nematic-isotropic liquid phase transition temperature (Tni).